The present invention relates to a method of optimising a pipe network. Particularly, but not exclusively, the invention relates to the optimisation of a pipe rehabilitation strategy for a pipe network such as a water supply network. More particularly still, the invention can provide a xe2x80x9cleast costxe2x80x9d acceptable strategy for the rehabilitation of a selection of pipes within a water supply network.
It is now conventional to use computer modelling in the design and operation of pipe networks. For instance, computer models of water mains networks are commercially available which provide a mathematical model of the physical properties of the network. Typically such a model will identify each pipe and other network elements (such as valves, pumps etc), giving the size, material and age etc (where this information is known) all of which may have an effect on performance of the network. Where precise details of pipe elements are not known assumptions may be made.
A water mains network will typically be divided into a number of separate district meter areas (DMAs) which will be separately modelled within the network model as a whole. A typical network will have half a dozen or so DMAs each having a designated source, which may be a real source such as a service reservoir, or a pseudo-source such as a trunk main. Alternatively, sources may be located further back upstream on trunk mains, with the modelled DMAs being supplied via branch mains off the trunk mains.
Within the network, and within each DMA, the network model will identify xe2x80x9cnodesxe2x80x9d. The concept of nodes will be familiar to those skilled in the art of network analysis. Nodes are designated by the network model builder, or the original geographical survey of the physical network on which the model is based, and includes such things as pipe junctions, pressure points, and demand points (typically models for residential areas will have 20 to 30 houses allocated to each node). The points where individual service pipes for single properties branch from the network would not generally be considered as network nodes (although there may be exceptions to this, for instance for models that cover sparsely populated rural areas).
The information provided by a network model can be used in the analysis of the performance of the network. Software packages are commercially available which can perform a number of operations on a network model, including calculation of such properties as pressure gradients and flow directions and rates etc. The core of such programs is a mathematical solver often referred to as a xe2x80x9chydraulic enginexe2x80x9d. In addition to the hydraulic engine the software will also include a front end to interface with the user, a back end and appropriate additional modules such as display, graph and import/export engines. Such software packages will hereinafter be referred to as xe2x80x9cnetwork analysis toolsxe2x80x9d.
One of the functions that conventional network analysis tools can provide is to predict the effect on a network (or on a DMA or part of a DMA) of changes made to certain elements within that network. As such, network analysis tools can be used when planning a rehabilitation strategy. Typically the network planner will have a number of performance constraints which must be accommodated, such as ensuring that rehabilitated pipes do not introduce an unacceptable steep hydraulic gradient (i.e. too much loss of pressure per unit length along a pipe line) but within those constraints the planner will be able to make a number of choices on rehabilitation technique, and the size of any new pipe or pipe lining that might be installed to rehabilitate the existing pipe. The network planner will have details of contractors which offer appropriate rehabilitation services and associated costs. Costs will typically vary not just with the rehabilitation technique, and size of any rehabilitated pipe, but also with location and ground conditions. For instance, it will be more expensive to dig up a main road than a side road etc. Usually the network planner will be constrained to select from a pre-agreed list of preferred contractors which have submitted acceptable tenders. This information is referred to hereinafter as a contractor xe2x80x9ccost tablexe2x80x9d (regardless of whether or the information is in fact tabulated).
Normally, the planner will want to determine the cheapest acceptable strategy by reference to the relevant cost table. Where a single pipe or pipe element is to be rehabilitated performance predictions, and therefore design choices, are easily made. However, the process can be extremely complex where a number of changes are to be made at different parts of a network, many of which will impact on each other and on other parts of the network.
It is therefore an object of the present invention to provide a method of optimising a pipe network. In particular, it is an object of the invention to provide a method of determining a xe2x80x9cleast costxe2x80x9d acceptable solution when planning a pipe network rehabilitation strategy. The method according to the present invention is not, however, limited to optimising by reference to cost, but could be adapted to provide optimisation on the basis of other criteria. In addition, the invention is not limited in application to rehabilitation of existing networks but can be used to aid in the planning of a new network.
It is a further object of the present invention to provide an optimisation method which can be implemented by computer software either as an integral part of a network analysis tool (such as mentioned above) or as a discrete module which can be added to existing network analysis software to provide enhanced functionality.
According to a first aspect of the present invention there is provided a method of optimising a model of a pipe network with respect to a predetermined criteria, the method comprising modifying a starting proposal for a list of pipes within the network model which may be modified by performing the following operations:
i) selecting a first pipe from the pipe list which may be considered for modification;
ii) proposing a modification to the selected pipe which provides an incremental improvement in said criteria;
iii) performing a network analysis of at least one predetermined operating parameter of the network to predict whether a predefined operating limit of said operating parameter will be violated as a result of the modification;
iv) if said network analysis predicts a violation of said predefined operating limit, then rejecting the proposed modification and removing the respective pipe from consideration for any further modification;
v) selecting a next pipe from the pipe list which may be considered for modification and performing operations (ii) to (iv) on the selected pipe;
vi) repeating operation (v) until all pipes which may be considered for modification have been selected; and
vii) repeating operations (i) and (vi) until no pipes of the pipe list remain to be considered for further modification.
According to a second aspect of the present invention there is provided a method of determining the hydraulic significance of each of a list of pipes within a model of a pipe network, the method comprising:
i) performing a network analysis on the network model to determine the flow patterns through the network at a given time;
ii) counting the number of instances of each pipe occurring in a flow path between a source node defined by the network model and the boundary of the network model, and using the instance count for each pipe as the indication of the hydraulic significance of that pipe within the network, such that pipes with a higher instance count are considered to more hydraulically significant than pipes with a lower instance count.
According to a third aspect of the present invention there is provided a method of determining peak flow rate demands on pipes within a model of a pipe network, the method comprising:
a) totalling the peak flow for the whole network and distributing this across the network to give a network peak flow demand on each pipe;
b) deriving a local peak flow demand representative of the localised demand on each pipe of the network; and
c) combining the network peak flow demand with the local peak flow demand to arrive at a peak flow rate demand for each pipe in the network.
As will become apparent, the invention has a number of novel aspects which are combined in preferred embodiments but which can also be utilised independently.
Other preferred and advantageous features of the various aspects of the present invention will be evident from the following description.